Atoms- What do we know?• “Nothing exists except atoms and empty space:

everything else is opinion.” Democritus: 400 BC.

• Atoms in 1900:– Can be grouped into types: for example the

halogens: Fluorine, Chlorine, Iodine, Bromine– Different atoms have different atomic weights:

usually multiples of the atomic weight of Hydrogen– They are very small. If we model atoms as 1mm

grains of sand. Then the model of a REAL sand grain would be a cube, edge length 1-10 km.

– Up to half the distance Cambridge to Ely

Atoms – Views of scientists in 1890-1900• Actual existence of atoms controversial.

• Problems were philosophical and scientific

• Theory of gases (Maxwell and Boltzmann): “billiard ball” collisions of molecules based on Newton’s laws of motion.

• Apparent inconsistency with Laws of Thermodynamics:

• First law: Energy cannot be created or destroyed

• Second Law: Despite this, actual physical processes do lead to “degradation” of energy so that some is effectively lost.

• Therefore, assumption of existence of molecules (and therefore atoms) must be incorrect.

Electrical currents through gases at low pressure

• Several people, including Faraday, investigated currents through gases at low pressure.

• Geissler invented an improved mercury pump

• Lower pressures obtained by raising &

lowering mercury column.

• This lead to coloured discharges

depending on the gas – Geissler tubes:

• Further improvement in pumps lead to enormous advances in physics. “ The biggest revolution in the history of science…”

William Crookes

With much lower gas pressures, “Crookes Tubes” were produced in which something (particles or rays ?) travelled from the negative cathode) to positive (anode) in straight lines.When they hit the tube they caused a glow.

High voltage generatorCathode

Anode

Cathode Rays - PARTICLES OR WAVES??• 1871 Cromwell Varley suggested particles

• 1876 Golstein & Helmhotz : “cathode rays”.

• Showed that rays were deflected by magnets but thought they were electromagnetic rays like light.

• 1879 Crookes thought the “rays” were particles and other British scientists took this view.

• Hertz showed that electric field had no effect so it seemed that they were really rays. However his vacuum was not very good.

• Several workers noticed nearby wrapped photographic plates became fogged.

• Crookes complained to the manufacturers!

• So what are cathode rays?

Wilhelm RöntgenBorn 1845 in Lennep Germany

1894 Philipp Lenard showed cathode rays could penetrate very thin foils of metals. Evidence for “rays”?

Röntgen used tubes with lowerpressure in Nov1895. Tried to check whether rays would pass through glass.

Covered tube with cardboard and hoped to detect rays with a fluorescent screen.

But out of corner of his eye saw a glow from another screen.......X-rays!!

X-ray of Röntgen’s wife’s hand 22 12 1895

X-ray taken during Röntgen’s public lecture 23 1 1896

Henri Bequerel – one of a dynasty.

Father and grandfather were interested in minerals that glow in the dark

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Radioactivity

Radioactivity Winter 1895 Paris

• Henri Bequerel continued investigating minerals that glow in the dark after Röntgen’s discovery.

• Wondered whether any connections to X-rays.

• Exposed crystal to light then

• placed them under wrapped photographic plates.

• Plates darkened.

• Even in dull weather with

little light.

• Plates still darkened - so light

• was not necessary.

• “Radioactivity” – a term coined

• by Marie Curie, who devoted her career to this.

• J. J. ThomsonBorn 1856 Cheetham Hill Manchester. • Mother textile worker, Father antiquarian bookseller.

• Father died when J.J. was16. Changed course from Engineering to Maths.

• Then Cambridge (Trinity again).

• Started research some of which predated Einstein’s result that mass of charged particle increases with speed.

• 4 years later appointed Cavendish Professor.

• William Thomson first choice but again refused.

• Later became Master of Trinity - first Master not a clergyman.

• Also first Cambridge Prof of Physics to originate from lower middle classes of North of England

Beta particles - electrons

Several workers showed that cathode rays can be deflected by magnets.

Hertz and Philipp Lenard found that the rays were not deflected by an electrical field.

But JJT showed that cathode rays can be deflected electrically.

He had tubes with a lower pressure.

Schematic of J.J.T’s apparatus (original in Cavendish Museum)

•By using equations derived from Faraday, JJT calculated the ratio of the charge on each particle, e to its mass, m; (that is e/m).

•He found e/m was about 2000 times larger than expected.

•The mass of the electron is about 2000 times smaller than the mass of the smallest atom.

•When he announced this at the RI listeners thought he was pulling their legs

Pieter Zeeman• Born in a small village in Zeeland

• Assistant in University of Amsterdam

• Decided to repeat Faraday’s attempts to measure changes in spectrum of a flame.

• Also wished to contradict Maxwell’s view that energy of light could not be altered by magnetic forces.

• Studied sodium light from a Bunsen flame in a magnetic field.

Spectrum of sodium light

• Zeeman saw a very small broadening of two closely spaced yellow lines.

• Hendrik Lorentz applied his theory and obtained a value for e/m - also very high.

• Published in 1896, so this value was known to J.J.

• The two values for e/m for an electron outside the atom & one inside the atom corresponded.

What does the electron tell us about atomic physics?

• The electron is clearly the unit of electric charge and may be involved in electrical conduction in metals and electrolysis.

• The electron is involved in oscillations within the atom which give rise its optical spectrum.

• Outside the atom, electrons can exist alone as cathode rays.

• Nobel prizes

1901 (first) Wilhelm Rontgen

1902 Henri Becquerel, Marie and Pierre Curie

1904 Philipp von Lenard

1905 Pieter Zeeman and Hendrik Lorentz

1906 J.J. Thomson

Centenary celebrations in Germany, France and UK

1900, Following J.J., what is the atom?

• Clearly atoms are no longer inviolable.

• They can be split apart to yield a particle only about 1/2000th the mass of the lightest atom, Hydrogen.

• Motion of electrons along a wire can provide a model for electric currents.

• Thus removing old definitions of electricity :

• “Imponderable fluid” and “Electric virtue”

• (William Gilbert court physician to Q. Elizabeth I)

• Note. J.J. didn’t call them electrons – he called them corpuscles.

• “Electron” from Greek word for amber

Ernest Rutherford

Cambridge 1895-1898• Rutherford: Cambridge University’s first Research

student.

• J.J. & Mrs J.J. welcomed him warmly.

• Not so other laboratory Physicists.

• Gained Street Cred. via his wireless devices.

• News of X-rays reached Cavendish. J.J. and R. start work on electrification of gases by X-rays.

• R. moved to Radioactivity after Becquerel’s work.

• Rutherford identified two types of radiation:• α rays: easily absorbed e.g. by paper, air.• β-rays: long range – greater penetration.• Rutherford also aware of a third type of

radiation, subsequently named γ-radiation by Paul Villard in 1900.

Manchester - Geiger and Marsden :scattering of α particles by nucleus

Glass tube filled with Radium “emanation”

Lead screen

Foil of various metalsLeadGoldPlatinumTinSilverCopperIronAluminium

Zinc Sulphide screen to show scintillations

Microscope

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Rutherford• Discovery of the (electrically negative) electron within

atoms raises a problem.

• Since the atom is electrically neutral, where is the positive charge?

• J.J. favoured a ‘plum pudding’ with charges spread out.

• Rutherford (in Manchester) set up anexperiment with α-particles (Heliumnuclei) from a radioactive source. • Most particles went straight through the thin gold foil.....but some bounced back!

• Rutherford said it was ‘like firing• a 15 inch shell at a piece of tissue• paper and it came back and hit you.’

Rutherford’s diagram showing how alpha particles are deflected when they pass close to a heavy nucleus

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Dimensions of nucleus• Rutherford (eventually) calculated the size of

the nucleus: 10-15 m.• So volume of nucleus is 10-45 cubic metres• Volume of atom is 10-30 cubic metres.• Nucleus is 1015 times smaller than an atom.• Rutherford pictured this as “A gnat in the

Albert Hall”.• But the gnat’s density is enormous: a gnat-

sized gold particle weighs a million tonnes.

• Roughly the mass of the Albert Hall!

Bohr- Rutherford anatomy of an atom

• In the 1910s Ernest Rutherford and Niels Bohr devised a model for the atom.

• Electrons – electrically negative – surround a positive nucleus.

•The nucleus contains protons (+) and also (neutral) neutrons •(James Chadwick 1932).

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• Bohr- Rutherford model of electron absorption and emission

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Incoming radiation causes ejection of an electron to a higher quantum orbit.

Electron falling to a lower quantum orbit releases radiation

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Problems with the “Astronomical”model of the atom.

• Classical theory requires that a circulating charged particle emits radiation.

• So the circulating electron would lose energy continuously.

• And quickly spiral down into the nucleus.• Solution to the problem demands that

energy should not be emitted continuously.• Another opportunity for Quantum science!• If one young man (Einstein) could

revolutionise science, why not another?• Enter Niels Bohr.

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• Niels Bohr (again) made the quantum hypothesis the centre of his new theory (1913) of the structure of the atom.

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Bohr bolted together Newtonian dynamics and Quantum Theory.

Proposed that there are stable orbits – which do not spiral into the nucleus as classical physics demands.

Transitions between orbitals obey Planck’s Quantum law. Results agree with spectrum for Hydrogen.

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Beyond the Bohr atom

• Bohr’s hypothesis agreed with the spectrum of Hydrogen.

• H is the lightest atom - 1 proton in the nucleus and 1 orbiting electron,

• Bohr’s method failed for larger elements containing many electrons.

• But the key ingredient - “Quantisation of stable electron states and

• changes in energy was established.

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Louis de BroglieAfter detailed reflection on Einstein’s work,

“I suddenly had the idea, during the year 1923, that the discovery made by Einstein in 1905 should be generalised by extending it to all material particles and notably to electrons.”

E = mc2 (Einstein)

E = hν (Planck);

So simply coupling the two

equations we get:

E = hν = mc2

So electron “mass” has an

associated wave motion.

Particles become “Wavicles”22

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DeBroglie’s Harmonics

De Broglie had in mind wave patterns for a stretched string.• Rather than electrons occupying “orbits”- as Bohr had suggested, he thought that the lowest level electron state would be the fundamental mode of “vibration”.

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• Higher orbitals could be represented by harmonics.

De Broglie’s thesis supervisor and examiners had doubts about the validity of the work and consulted Einstein who was supportive: “I believe it is a first feeble ray of light on this worst of our physics enigmas.”

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Contemporary reaction to De Broglie

• In France... The thesis was published in full in a special edition of “Annales de Physique” in 1925.

• Outside France, wave-particle duality met a sniffy reaction: “ la Comédie Française”.

• (Dirac was not impressed.)

• Einstein reacted positively: drew De Broglie’s work to the attention of Erwin Schrödinger, a 38-year old Austrian (who had not read De Broglie’s papers).

• Schrödinger realised this paper covered similar ground to some of his earlier work and he was also attracted to the idea of harmonics.

• He talked about his ideas in an informal seminar but was told that he should try to construct a wave equation to replace “this rather childish way of talking”.

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Some reactions to De Broglie’s proposal: hν =E = mc2

• Roger Penrose in “The Emperor’s New Mind”:• “Thus, according to De Broglie’s proposal, the

dichotomy between particles and fields that had been a feature of classical theory is not respected by nature.

• Somehow, Nature contrives to build a consistent world in which particles and field-oscillations are the same thing.”

• Or, rather, her world consists of some more subtle ingredient, the words ‘particle’ and ‘wave’ conveying but partially appropriate pictures”.